Known processes for producing optically active 3-hydroxy-2-methylbutyrates from 2-methyl-3-oxobutyrates of the formula (1) by microbial asymmetric reduction include the processes using baker's yeast [A. Furuichi et al., Agric. Biol. Chem., 49, 9, 2563-2570 (1985); K. Nakamura et al., Tetrahedron Letters, 27, 3155 (1986); and K. Nakamura et al., Bull. Chem. Soc. Jpn., 62, 1179 (1989)] and the process using other yeast such as Candida albicans [H. Akita et al., Chem. Pharm. Bull., 31, 4376 (1983)]. However, these processes are not industrially advantageous processes in view of their low diastereomer selectivity and/or enantiomer selectivity. Further, microbial asymmetric reduction of 2-methyl-3-oxobutyrates with bacteria has not been known heretofore in the prior art.